Since the advent of a resist for KrF excimer laser (248 nm), an image forming method called chemical amplification is used as an image forming method for a resist so as to compensate for sensitivity reduction caused by light absorption. For example, the image forming method by positive tone chemical amplification is an image forming method of decomposing an acid generator in the exposed area upon exposure to produce an acid, converting an alkali-insoluble group into an alkali-soluble group by using the generated acid as a reaction catalyst in the baking after exposure (PEB: Post Exposure Bake), and removing the exposed area by alkali development.
With respect to the developer for g-line, i-line, KrF, ArF, EB or EUV lithography, an aqueous alkali developer of 2.38 mass % TMAH (tetramethylammonium hydroxide) is being used at present as a general-purpose developer.
Other than the above-described developer, for example, Patent Document 1 describes a developer for effecting development by dissolving the exposed portion of a resist material whose polymer chain is broken upon irradiation with radiation to reduce the molecular weight, wherein the developer contains at least two or more members of an acetic acid group, a ketone group, an ether group and a phenyl group and at the same time, has a molecular weight of 150 or more. Also, Patent Documents 2 and 3 describe a developer for effecting development by dissolving the exposed portion of a resist material containing a specific resin having a fluorine atom, wherein the developer is selected from a supercritical fluid, a halogenated organic solvent and a non-halogenated organic solvent.
However, as the miniaturization of a semiconductor device proceeds, it is actually very difficult to find an appropriate combination of a resist composition, a developer and the like for forming a pattern with overall good performance and furthermore, a pattern forming method using those. In this respect, improvements are being demanded.
Along with the miniaturization of a semiconductor device, there is becoming shorter the wavelength of the exposure light source and higher the numerical aperture (higher NA) of the projection lens, and an exposure machine using an ArF excimer laser having a wavelength of 193 nm as a light source has been so far developed. As commonly well known, these factors can be expressed by the following formulae:(Resolution)=k1·(λ/NA)(Depth of focus)=±k2·λ/NA2 wherein λ is the wavelength of the exposure light source, NA is the numerical aperture of the projection lens, and k1 and k2 are coefficients related to the process.
A so-called immersion method of filling a high refractive-index liquid (hereinafter sometimes referred to as an “immersion liquid”) between the projection lens and the sample has been conventionally advocated as a technique for raising the resolution.
As for the “effect of immersion”, assuming that NA0=sin θ, the above-described resolution and depth of focus in the immersion can be expressed by the following formulae:(Resolution)=k1·(λ0/n)/NA0 (Depth of focus)=±k2·(k0/n)/NA02 wherein λ0 is the wavelength of exposure light in air, n is the refractive index of the immersion liquid based on air, and θ the convergence half-angle of beam.
That is, the effect of immersion is equal to use of an exposure wavelength of 1/n. In other words, in the case of a projection optical system with the same NA, the depth of focus can be made n times larger by the immersion. This is effective for all pattern profiles and can be combined with the super-resolution technology under study at present, such as phase-shift method and modified illumination method.
For example, in Patent Documents 4 to 7, a method of demagnifying the space dimension of a resist pattern more than the resolution limit is disclosed as the technique for further enhancing the resolution.
Also, in Patent Document 8, a developer composed of an organic solvent having a lower polarity than an aqueous alkali solution is described as a developer for dissolving and removing the unexposed portion of a resist film formed of a resin whose polarity increases upon irradiation with radiation.
These methods all make use of the characteristics of the chemical amplification-type resist material and utilize a technique where a resist pattern containing an acid generator capable of generating an acid is formed by the normal lithography technology, a material that reacts in the presence of an acid to form a crosslinked layer insolubilized in a developer (sometimes referred to as a “crosslinked layer-forming material”) is deposited thereon, and through an additional treatment step such as heating, the acid is diffused into the crosslinked layer-forming material from the resist pattern to form a developer insoluble layer at the interface between the resist pattern and the crosslinked layer-forming material, thereby thickening the dimension of the resist pattern and effectively demagnifying the trench or hole dimension of the resist pattern.
However, in all of these techniques, a larger amount of an acid is utilized and therefore, an additional step such as surface treatment with an acidic substance or another exposure step or furthermore, a heat treatment step at a high temperature is required in some cases, making it difficult to ensure a sufficiently large amount of an acid that is a component effective for forming a developer insoluble layer at the interface with the crosslinked layer-forming material.
In order to solve this problem, Patent Document 9 takes an approach of previously increasing the amount of an acid generator added in the resist to increase the amount of acid diffusion, but in this case, the increase in the amount of acid diffusion involves another problem of deterioration of the resolution, particularly, the exposure latitude, of the resist pattern. The relationship between the increase in the amount of an acid added in the resist and the deterioration of exposure latitude is disclosed, for example, in Non-Patent Document 1. Also, the relationship between the increase in the amount of acid diffusion and the deterioration of exposure latitude is disclosed, for example, in Non-Patent Document 2.
In addition to these circumstances, when the space dimension of an actual resist pattern is demagnified using the method disclosed above, a resist residue (scum) remains on the wafer. Improvement of this problem is being demanded.
Patent Document 1: JP-A-2006-227174 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
Patent Document 2: JP-T-2002-525683 (the term “JP-T” as used herein means a “published Japanese translation of a PCT patent application”)
Patent Document 3: JP-T-2005-533907
Patent Document 4: JP-A-5-241348
Patent Document 5: JP-A-10-73927
Patent Document 6: JP-A-2001-19860
Patent Document 7: JP-A-2004-61668
Patent Document 8: JP-A-2000-199953
Patent Document 9: JP-A-2003-249437
Non-Patent Document 1: Journal of Vacuum Science and Technology B, Vol. 12, No. 6, 3863 (1994)
Non-Patent Document 2: Journal of Vacuum Science and Technology B, Vol. 24, No. 1, 316 (2006)